It is known in the art to employ organic polymers having low dielectric constant in electronic applications such as integrated circuit chip substrates and circuit boards. The requirements for such applications include low CTE (<120 ppm/° C.), low flammability (typically V-0), low dielectric constant (<4.0) (Dk), low electrical dissipation factor (<0.03) (Df). By far the most common resin in commercial use is mineral filled epoxy resin containing halogen flame retardant.
Electronic circuits are exposed to repeated changes in temperature and humidity during processing and in use. Because the constituent components differ in CTE and moisture uptake, delamination of electrical contacts may occur. Furthermore, moisture may induce corrosion. Reliability of electronic circuits can be improved if the dielectric material employed therein exhibits a CTE close to that of silicon (3-5 ppm/° C.) or copper (˜18 ppm/° C.), low moisture absorption, and improved toughness. The epoxy dielectrics in widespread commercial use are widely regarded in the art as unsuitable for the next generation of electronic circuitry which is characterized by higher circuit density and smaller electrical contacts (which means more highly concentrated mechanical stress).
The electrical performance of electronic components is affected by the properties of the dielectric material. When a high frequency electronic signal propagates through a conductor (for example, printed circuits on a circuit board), an electromagnetic field permeates into the organic polymeric material adjacent to the conductor. The interaction between the organic polymeric material and this electromagnetic field affects the propagation properties of the signal. These interactions are especially important for high circuit density and high frequency applications. For these reasons, the dielectric properties of the organic polymeric material are important. In particular the dielectric constant determines the speed of signal propagation through the circuit and affects signal cross talk between circuits, and the dissipation factor determines signal loss.
With each new generation of products circuit density increases while signal frequencies increase, thereby placing ever greater demands upon the organic polymers employed as dielectric material therein. Ever lower dielectric constant, low dissipation factor must be coupled with precisely controlled CTE and toughness.
In addition, flammability is an on-going concern halogen-containing flame-retardants commonly employed in commercial epoxy compositions in order to provide Underwriters Laboratory V-0 ratings are becoming subject to ever more stringent environmental controls. Thus a candidate material for the next generation product must also meet the V-0 requirement without use of halogen containing flame-retardants.
An additional pragmatic requirement is that any candidate material must exhibit the processibility of an epoxy thermoset in order to employ existing manufacturing equipment. This is mostly a requirement concerning flow and formability, particularly in vacuum forming.
Japanese Kokai JP 2003-060352 discloses a multilayer printed wiring board employing heat resistant films formed from polymers such as polyaramids, aromatic polyimides, poly parabenzimidazole resin, poly parabenzoxazole resin, and poly parabenzthiazole resin. Epoxies are employed to bind the films to copper. The resulting printed wiring board is said to exhibit mechanical strength, heat resistance, adhesion strength, durability dimensional stability, and insulation reliability enabling high density of a wiring. Japanese Kokai JP 2002-264267, JP 2002-144475, JP 2002-076641, JP 2002-076619 and JP2002-219789 further describe various aspects and embodiments of the printed wiring boards thereof.
Japanese Kokai JP2002-363283 and JP2002-060490 disclose a polyimide resin, which is characterized by low CTE (13-29 ppm) and low dielectric constant (2.57-2.96). The polyimide is prepared from a polyamide acid precursor. The polyamide acid is obtained by reacting in an organic solvent acid anhydrides consisting of an anhydride (either pyromellitic acid anhydride or 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride), and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane, with 2,2′-disubstituted-4,4′-diaminobiphenyls and another aromatic diamine selected from 2,2-bis (4-aminophenoxyphenyl)propanes, 1,1-bis(4-(4-aminophenoxy)-3-t-butyl-6-methylphenyl)butane and 2,2-bis(3-amino-4-methylphenyl)diisopropylbenzenes.
Japanese Kokai JP2003-192788 discloses a polyimide copolymer derived from pyromellitic anhydride useful for flexible printed wiring boards. The copolymer disclosed is characterized by heat resistance, insulating resistance, chemical resistance, low modulus of elasticity, and low CTE (15-25 ppm at 100-200° C.). Japanese Kokai JP,02/014406,A1 discloses an improvement thereto characterized by a CTE 10˜20 ppm @100˜200C, which is close to the CTE of metal foil so that metal foil laminated material offers good coplanarity without warpage or other damage such as cracks, and delamination.
Auman et al, “Fluorinated polyimides for interlayer dielectric applications: Tailoring of properties via copolymerization,” Book of Abstracts, 211th ACS National Meeting, New Orleans, La., Mar. 24-28 (1996), POLY-016. Publisher: American Chemical Society, Washington, D.C. discloses rod-like fluorinated polyimides for interlayer dielec. (ILD) applications. Copolymerization is disclosed as a means for tailoring properties of rigid polyimides. The rod-like structures disclosed therein exhibit very low in plane (CTE), with anisotropic dielectric properties. Also disclosed is modification of the highly rod-like polyimide incorporation of a more flexible fluorinated comonomer, 6FDA, at various levels to increase CTE and balance dielec. const.
Feiring et al ,“Synthesis and properties of fluorinated polyimides from novel 2,2′-bis(fluoroalkoxy)benzidines”. Macromolecules (1993), 26(11), 2779-84 discloses polyimides prepd. from 2,2′-bis(fluoroalkoxy)benzidines and several dianhydrides. The diamines, containing. OCF3, OCF2CF2H, and OCF2CFHOC3F7 groups, were obtained from the corresponding 3-(fluoroalkoxy)nitrobenzenes by reduction to hydrazo derivatives, followed by benzidine rearrangements. Polymers prepared from 2,2′-bis(trifluoromethoxy)benzidine showed a combination of low dielectric constant, moisture absorption, CTE and high thermal stability.
Marchetti et al, U.S. Pat. No. 4,675,350 discloses an oligomeric polyimide-curing agent for epoxy via a polyamic acid precursor. The resulting cured epoxy is said to exhibit higher glass transition temperature, higher toughness, and improved chemical resistance.
Ohmae et al, U.S. Pat. No. 5,047,479 discloses ethylene polymer functionalized with maleic anhydride, and partially cross-linked by polyamide to form a thermoplastic material.